[Info-vax] Best way to get what I can off a failed VMS Disk?

[Jan-Erik Söderholm]

Den 2020-10-24 kl. 23:54, skrev Phillip Helbig (undress to reply):
> In article <rn1qo9$oms$2 at dont-email.me>, Simon Clubley
> <clubley at remove_me.eisner.decus.org-Earth.UFP> writes:
> 
>>> Would forming a shadow set be slower than an offline disk duplicator?
>>>
>>
>> Will forming a shadow set cause random I/O all over the failing
>> disk or will the disk be read exactly once from start to finish
>> in strict sequential order without any additional I/Os ?
> 
> There are shadowing experts here who might reply.  I believe it moves
> sequentially through the source, writing to the target, and keeping
> track of what portions were updated since the begin of the copy, then
> after the initial pass updates those sections, and so on, until it
> converges.  I forget how large these sections are, but there is a bitmap
> (used for MINICOPY and MINIMERGE as well) which keeps track of whether
> that section needs to be updated or not, and if so the whole section is
> copied.  (This is a balance between writing too much unnecessary stuff
> and having a bitmap as large as the disk itself.)
> 

Why would a single bit in the bitmap point to a smaller part of the disk
than the smallest writeable part (a block, a cluster or whatever)? Can
you write anything less than a 512 byte block?

But anyway, the whole shadowing thing depends on that the source disk
can be mounted, and it couldn't, if I understood correctly.